Poly envelope offers a lightweight yet durable construction that resists moisture to protect your valuables from the elements. Heat-welded seams make the envelope more durable and long-lasting than traditional paper envelopes. See-through design allows easy identification of the contents. Top flap seals securely with a string-and-button closure. Expandable 1-1/4" gusset allows items of various thicknesses to be stored inside. Reusable envelope is made of durable polypropylene and is moisture-resistant, stain-resistant and tear-resistant. Poly material is acid-free, PVC-free, lignin-free and archival-safe.